A biosensor is a device that employs a biological sensing element to produce an electrical or optical signal in proportion to the concentration of a particular substance of interest, often referred to as the analyte. In order for a biosensor to operate, it must make use of a chemical reaction or measurable property change. Since many medical conditions are diagnosed or monitored by measuring the concentration of certain characteristic analytes, modern medicine relies heavily on biosensors to accurately direct the course of treatment.
In vivo optical biosensors, which operate based on changes in the absorbance and/or reflectance of certain diagnostic wavelengths of light incident on some part of the patient, can offer high accuracy while being much less invasive than alternative methods that require a sample to be taken, or that utilize electrodes which penetrate the skin.
For example, the pulse oximeter is a widely-used, highly reliable, and minimally invasive in vivo optical biosensor. Pulse oximetry is most often used to monitor blood oxygenation levels by shining light through a fingertip and calculating the ratio of the absorbance of light at 660 and 910 nm by the bloodstream. Oxygenated and deoxygenated hemoglobin absorb differently near 660 nm. Specifically, oxygenated hemoglobin is red, which means it weakly absorbs red light. In contrast, deoxygenated hemoglobin absorbs strongly at 660 nm, giving it a blue color. Both states absorb similarly at 910 nm, so the ratio of absorbance values can be used to find the percentage of hemoglobin molecules that are oxygenated. However, in vivo optical biosensing has, to this point, generally been limited to biomolecules, like hemoglobin, that happen to have discernable optical features within the “optical window” of light wavelengths that can pass through the skin without being overly absorbed or scattered. Because of this interference, primarily caused by the water content of living tissue, as well as other confounding species that may absorb at nearby wavelengths, many important analytes, including glucose, have resisted attempts to be detected through minimally invasive, in vivo optical biosensors.
Glucose monitoring itself is extremely important in the course of treatment for diabetes mellitus. Diabetes is a widespread, chronic condition with significant public health implications. In particular, this metabolic disorder can lead to severe medical complications and consumes a large amount of health care resources every year. According to the American Diabetes Association, there are an estimated 20.8 million diabetics in the United States, representing 7% of the population, with this number only expected to increase in the future. The total annual economic cost in 2002 was estimated to be $132 billion, including $92 billion in direct medical costs and another $40 billion for indirect costs attributed to disability, work absenteeism, and premature mortality. Complications of diabetes can include: heart disease, stroke, high blood pressure, blindness, kidney disease, nervous system damage, amputations, dental disease, and sexual dysfunction. Essential to preventing these complications is a treatment regimen that maintains blood glucose concentrations within normal limits. However, the current solution of “fingerstick” testing of the blood glucose from capillary blood four or five times a day to calibrate insulin injections is a significant burden on patients. The pain and inconvenience of testing reduces compliance and can lead to inadequate control of blood sugar levels. Additionally, dangerous spikes or dips in blood sugar between tests may go unnoticed, and levels are unknown during times when testing is not feasible, such as when the patient is sleeping.
Thus, there remains a need for a continuous monitoring system that can display and record glucose levels in real time, sound visual and/or audible alarms during hyper- or hypoglycemic events, and work in a feedback loop with an insulin pump. Although significant progress has been made in improving commercial realtime implantable monitors, for example, systems that use data collected amperometrically from implanted electrodes, many barriers still remain that prevent them from enjoying more widespread use. Considerations of cost, reliability, invasiveness, and lifetime justify a continued search for alternative continuous sensing methods. Since glucose lacks significant optical absorptions in the range of wavelengths skin is transparent to, attempts directly measure it through in situ optical techniques have been difficult.
Considering the above-mentioned concerns, there is a need for a system capable of in vivo biosensing that will provide a high level of patient compliance.
In particular, it is clear that there remains a need in the art for an improved system to aid in the effective treatment of such diseases as diabetes.